The present invention relates in general to hydrophilic coatings and, more particularly, to optionally antimicrobial, hydrophilic coatings having reduced or low total solids contents.
2. Description of Related Art
Condensing heat exchangers for micro or zero gravity applications rely on hydrophilic and antimicrobial, hydrophilic coating systems to "wet out" condensed water for subsequent collection. In particular, the antimicrobial, hydrophilic coating systems inhibit microbial proliferation and cause wetting and wicking, thereby inducing condensate in the condenser to form a thin spreading film in the coating that can readily be collected. This thin film is collected through "slurper" holes into a gas-liquid phase separator which keeps water droplets from being entrapped in the gaseous stream from which it was removed. (See U.S. Pat. No. 3,868,830.)
U.S. Pat. No. 3,658,581 to Paul et al., which is incorporated herein by reference, discloses a high solids hydrophilic passive coating which facilitates wetting and wicking on heat transfer surfaces. The hydrophilic qualities of this coating result from the chemical polarity of uncoated silica or calcium silicate dispersed in a non-crystalline binder and from the capillary attraction of the water molecules for one another. The silica and calcium silicate particles have a polar attraction to hydroxyl ions in the condensate water and thereby pull the water to the coating, known as wetting. Wicking or capillary attraction then comes into play as the water being drawn into the coating pulls additional water along with it.
Due to the high solids content of the Paul et al. hydrophilic coating, application of the coating can be difficult and time consuming. In addition, after the coating is heat cured, it is highly susceptible to cracking, flaking and subsequent particle generation. A preferred slurry formulation of Paul et al. comprises: 125 parts by weight silica; 12 parts by weight zinc oxide; 222 parts by weight potassium silicate; and 500 parts by weight water. (See Column 1, lines 52 to 57.) Another preferred slurry formation disclosed in Paul et al. comprises: 100 parts by weight silica; 100 parts by weight lead borosilicate glass frit; 5.8 parts by weight boric acid; 5.2 parts by weight potassium hydroxide; 3.9 parts by weight sodium silicate; and 150 parts by weight water. (See Column 2, lines 24 to 40.) These slurries deposit a relatively thick (1 to 10 mil) coating that tends to "pool" and deposit, by dip application and cure (as hereinafter defined), approximately 23 to 28 milligrams (mg) of cured coating per square inch (in.sup.2) of coated or dipped material. The tendency of these slurries to "pool" is magnified when coating hardware with a constrained geometry. As a result, the Paul et al. coating, after heat cure, is prone to cracking, flaking and particle generation. By way of example, prior Space Shuttle Temperature and Humidity Control Fan Separator failure analyses have attributed water separator flooding and water carry over anomalies to plugging of the water separator pitot tube with hydrophilic coating particles generated from the upstream condensing heat exchanger. With longer term missions on the horizon, the elimination or reduction in the need for system maintenance is required. Temperature and Humidity Control System downstream contamination would be extremely detrimental for an extended mission since routine maintenance would be the primary way to circumvent downstream water separator flooding and water carry over anomalies.
In addition to the above, the application of such a high solids slurry must be accomplished within a very short time period (approximately 15 minutes) so as to avoid excessive agglomeration and settling of the slurry solids. In a recent coating episode of a Space Station predevelopment condensing heat exchanger the inability to accomplish the coating procedure within the noted time period led to a deposited solid plug of hydrophilic coating which impeded air flow through the heat exchanger and which lead to costly and time consuming rework.
Moreover, due to the porous characteristics of the Paul et al. coating it can potentially entrap organic, inorganic and microbial contaminants. As a result, the coated heat transfer surfaces, during extended periods of operation, constitute ideal locations for microbial proliferation which can reduce the hydrophilic properties of the coating, plug slurper holes, and corrode the heat transfer surfaces, thereby decreasing the heat transfer efficiency of the condenser. Additionally, if these microbes become air borne, they can be inhaled and cause adverse health effects and they can result in odor generation in the gaseous stream exiting the condenser. As a result, microbial proliferation can lead to heat transfer reduction, along with health and comfort concerns in relation to condensers, especially condensers which operate within a closed environment.
Since the Paul et al. coating has generally only been utilized for about 7 to 10 consecutive days, microbial proliferation has not been a great concern. After use, these condensers and the heat transfer surfaces would dry, thereby inhibiting microbial proliferation. However, in applications where the condenser will be utilized for extended periods of time, such as on a space station for 10 years or more, microbial proliferation becomes a major concern.
U.S. Pat. No. 5,264,250 to Steele et al., which is also incorporated herein by reference, partially addresses the problems associated with the Paul et al. coating. In particular, Steele et al. disclose a coating having hydrophilic and biocidal characteristics. However, the Steele et al. coating, like the coating disclosed in Paul et al., has a high solids content. In particular and as set forth above, preferred slurry formations disclosed in Steele et al. deposit, by dip application and cure, approximately 23 to 28 mg of cured coating per square inch of the coated or dipped material.
It is therefore an object of the present invention to provide a low solids, optionally antimicrobial, hydrophilic coating having improved coating properties.
It is another object of the present invention to provide a low solids, optionally antimicrobial, hydrophilic coating that, upon cure, is less prone to cracking, flaking and particle generation.
It is a further object to provide a condensing heat exchanger whose heat transfer surfaces are coated with a low solids, optionally antimicrobial, hydrophilic coating.
It is yet a further object to provide a method for coating heat transfer surfaces of a condensing heat exchanger with a low solids, optionally antimicrobial, hydrophilic coating.